Huge Explosion Reveals the Most Massive Star Known

All supernova explosions are violent affairs, but this one
takes the cake. Astronomers have spotted a new type of extremely bright cosmic
explosion they think originates from an exceptionally massive star.

This breed of explosion has been long predicted, but never
before seen. Like all
supernovas, the blast is thought to have marked the end
of a star's life. But in this case, that star may have started out with 200
times the mass of the sun.

The supernova in question, SN2007bi, was observed in 2007 in
a nearby dwarf galaxy. Scientists knew at once it was something
different because it was about 50 to 100 times brighter than a typical
supernova.

"It was much brighter, and it was bright for a very
long time," said researcher Paolo Mazzali of the Max-Planck Institute for
Astrophysics in Germany. "We could observe this thing almost two years
after it was discovered, where you normally don?t see anything anymore."

After analyzing its signature, astronomers published a paper
in the Dec. 3 issue of the journal Nature confirming that it matches
theoretical predictions of a so-called pair-instability supernova.

"There were some doubts that they existed," said
astronomer Norbert Langer of the University of Bonn in Germany, who did not
work on the project. Langer wrote an opinion essay on the finding in the same
issue of Nature. "There were severe doubts that stars that massive could
ever form in the universe. Now we seem to be very sure that there was a star
with 200 solar masses."

In a pair-instability supernova, the star has neared the end
of its life and exhausted its main supplies of hydrogen and helium, leaving it
a core of mostly oxygen. In smaller
stars, the core continues to burn until eventually it is all iron, then
collapses in a Type II supernova, leaving behind a remnant black hole or
neutron star.

But in the case of an extremely massive star, while its core
is still made of oxygen, it releases photons that are so energetic, they create
pairs of electrons and their anti-matter opposites, positrons. When the matter
and antimatter meet, they annihilate each other. This reaction reduces the
star's pressure, and it collapses, igniting the oxygen core in a runaway
nuclear explosion that eats up the whole star, leaving no remnant at all.

The discovery of this rare type of supernova suggests that a
few stars actually can grow into such large behemoths ? which has long been a
topic of debate.

"I was never a believer in very massive stars,"
Mazzali told SPACE.com. "Seeing something like this explode means these
things exist. This is a fairly new development in the formation of stars."